Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 43

GAS

LIQUID

SOLID

Materials consisting of single elements or compounds can exist in three well‐defined physical states: gas, liquid, and solid. These states depend on temperature and pressure.

A “gas” is a state of matter in which the atoms or molecules are separated by a large distance. Molecules in the gaseous state vibrate and move freely to occupy the volume made available. A related term is “vapor,” which technically refers to a material that is in the gaseous state at conditions in which it can also exist in the liquid or solid state (i.e. below its critical temperature, described further later in the text). Often, the terms “vapor” and “gas” are used synonymously.

A “liquid” is, in general, a condensed state of matter in which the particles are irregularly spaced (or, in other words, there is no long‐range order); that is, they are amorphous in nature. Unlike gases, liquids are generally not easily compressible; however, they flow easily.

On the other hand, under such basic classification schemes, “solids” are materials in which the particles are regularly spaced and form repeating structures in three dimensions; that is, they are crystalline in nature. Solids generally retain a fixed volume and shape and do not flow easily.

The outcome of such classification leads to some interesting areas to consider. For example, amorphous materials, such as traditional glasses, which are usually considered solid materials in everyday use, are classified as supercooled liquids (albeit this is debatable), and materials like liquid crystals, amorphous metals, and supercritical gases fall somewhat between the phase definitions. In addition, a fourth state, i.e. “plasma,” is introduced to describe matter at very high temperatures and pressures where the atoms start to break down resulting in a mixture of neutral atoms, free electrons, and charged ions. The sun, lightning, and beautifully colored auroras are examples of plasmas in nature.

Significant progress has been made in understanding the states and their boundaries, particularly for metals, metallic alloys, and ceramics (Reed‐Hill and Abbaschian 1992; chapter 10). The fluid states (liquid, gas, and plasma), though particularly important for process industries and for specific applications, are often not discussed when dealing with the compositions, physical metallurgy, and engineering mechanics revolving around microstructure–property relationships. Understanding the solid state is, however, of the utmost practical importance for all engineering applications for metals, alloys, and ceramics.

Due to the complex nature of materials, the modern‐day classification of states often relies on a more sophisticated system that focuses on the elastic response of the material. At a high level, in this scheme, solids respond to shear stress by the momentary relative motion of adjacent layers of particles whereas fluids undergo continuous relative motion or flow (DeVoe n.d.; chapter 2). This difference between states depends not only on temperature and pressure, but also on the timescale of observation. Time dependency will be a critical aspect explored throughout this text.